If last week's call-sign was "I agree with Nick", this one's will be "Nick's wrong". The Liberal Democrat leader's volcanic eruption in the opinion polls is so dramatic that both main parties are flailing around to respond. By the chance of drawn straws, Clegg will take centre-stage in the next debate. David Cameron and Gordon Brown will try to squeeze him to death. Whether or not they succeed will be the big story of the next seven days.

Since the debate is meant to be on foreign affairs, it is also clear where the new lines of attack on Clegg will focus. He has already been beaten up over his proposed abolition of the child trust fund, his "not quite an amnesty" plan for illegal migrants, and his old enthusiasm for the trouble-struck euro. Now the two big parties will come at him over not replacing Trident, for being too pro-European, and too nice to foreigners.

Electorally, the Tories will come at him hardest, because they have most to lose. A strong Lib Dem performance is hard to translate into many more seats but can easily deprive Cameron of an overall victory. Tory strategists are worried.

That's why Brown looked so cheerful today as he spoke of the election being wide open. The Lib Dems could help keep him in power, and have already halted the sense of inevitability about a Cameron victory. As one gleeful Labour cabinet minister said today, the Tories could have sealed the deal this week, but thanks to Clegg they haven't.

Yet, Labour has its problems, too. Although there are not many Labour-Lib Dem marginals, an increased vote for the Lib Dems at Labour's expense could be fatal for Labour in the 100 or so key Labour-Tory marginals. So we have seen a hilarious range of Labour responses, from the warmth of Alan Johnson towards a hung parliament, to Alistair Darling's brusque dismissal of the Lib Dems as "loopy". Brown tried to straddle both positions today, saying in effect, "they've got terrible policies but I love them to bits".

How should the Lib Dem leader respond? The next few days could smash the mould of British politics, or prove that his early success was a flash-in-the-pan, one-week campaigning wonder before the serious choice begins. And the reason we should hope for a strong second Clegg performance is less to do with psephology than principle. Particularly on foreign policy, there are truths that need to be heard. Today, he seems the man to tell them.

The first concerns the Middle East. Having been right about Iraq, the Lib Dems have little more to add on that subject, but would do well to remind British voters why we have been so bloodily entrenched in the region. We have become blandly forgetful about our own history. We forget the imperial takeover of much of the region, the air-bombing of Iraqi villages, the overthrow of Mossadeq in Iran, the close co-operation with brutal dictators, the CIA and oil companies, and even the illegal Suez war. We may choose to forget; people in the region haven't. We can't have an honest conversation without remembering what we forgot.

In Afghanistan, where we are supporting a corrupt and unpopular government against even worse people, let's remember where it all started – with western support for Islamist extremists whose only attraction was they hated the Russians even more than they resented us. Where Washington led, again and again, we innocently followed. So when Labour and the Tories turn sneeringly on the "naive" Lib Dems, wouldn't it be good to be reminded of some of the terrible blunders of postwar foreign policy the other two parties were responsible for?

That doesn't add up to the immediate exit from Afghanistan that many people would like: life is more complicated. But if the Lib Dems have a starker analysis of the failures in Iraq and a sharper timetable for an Afghan withdrawal, that's likely to be popular, not a "gaffe".

Europe is harder for them, because they have been clearly more federalist in their instincts than the majority of voters. Already there have been some unpleasant attacks on Clegg for his multinational origins – Dutch and Russian, with a Spanish wife – and the implication that behind the attractive face lurks a devilish European. But again, this week, I hope he comes out fighting.

Both Labour and the Tories have found it convenient not to talk too much about Europe in this election. Labour doesn't because its people know that with high levels of EU migration, "Europe" is unpopular just now. The days of Tony Blair's frankly expressed Euro-enthusiasm are long gone. Brown dislikes those wearisome EU meetings, and doesn't care who knows it: Europeanism has become the policy that dare not speak its name.

On the Tory side, the reason they don't talk about Europe is because their policy is a mess, and is likely to blow up in their faces if they win an overall majority. Unlike Labour, the policy is firmly anti-federalist but has taken them to positions that will be unsustainable in office, and unpleasant political alliances they cannot be proud of. Meanwhile, a large number of their candidates, members and voters would like them to go much further and leave the EU entirely.

So Clegg ought to relish debate on Europe, and to come back at Labour for being so shame-faced and half-hearted about the policy it has adopted, while tearing into the inconsistencies and unanswered questions in Conservative policy.

Why, you may ask, would a Labour-supporting writer want to see Clegg on the offensive? Not just because at a particular percentage of support he might cause more problems for Cameron than for Brown: I don't think you can third-guess possible outcomes just yet. It's rather that we just haven't had a serious and nuanced debate about our place in the world for a long time – and Clegg is the person likeliest to open up these issues. If he goes on the attack, we will see other progressive people, including Labour, pile in – and the election campaign will have been far better for it.

Last time round, and the time before that, we had election campaigns fought on details of domestic policy, as if Britain was barely engaged in Europe or the rest of the world. Given what followed, that was dishonest. Now, with coffins and amputees returning from Asia, our economy bled white by an economic crisis that started in mid-America, and our readiness to deal with global warming intertwined with the rest of the EU, we simply cannot afford to spend the next fortnight ignoring foreign affairs. Clegg electrified the first full week of campaigning. We need him to do it again.